1. Field of the Invention
This invention relates to actuation systems for use in gas turbine engines and, more particularly, actuation systems for transferring movement of an external actuator into an interior of a gas turbine engine to actuate an internal sliding member or valve.
2. Description of the Prior Art
The development of this invention was precipitated by advanced versions of variable cycle gas turbine engines. Since the 1950's, there has been ongoing development of this type of engine for use in jet aircraft. In the variable cycle engine, relative amounts of air directed through a fan bypass cycle, as opposed to a combustion cycle, are varied under different operating conditions to improve engine performance. In one embodiment of this engine, airflow is controlled by a forward sliding valve system, called a variable area bypass injector (forward VABI), that is located in a passage between an inner and outer bypass duct, and is opened and closed to vary the amount of fan air flowing into the outer bypass duct and, therefore, bypassing the combustion cycle. An additional valve-like mechanism, called a rear variable area bypass injector (rear VABI), is provided at the end of the bypass duct for injecting bypass airflow back into the core engine flow. For a detailed description of this type of variable cycle engine, the reader is referred to U.S. Pat. No. 4,068,471, issued Jan. 17, 1978, assigned to the same assignee as the present invention, and the disclosed material of which is incorporated herein by reference.
The forward and rear bypass valves are designed to be operated by an actuating mechanism that is capable of transferring axial motion from external actuators through an outer casing of the engine to the internally mounted valve. In prior art mechanisms, this type of transfer of motion is often accomplished by multiple radial shafts extending through the engine casing. Mechanisms capable of driving a multiple radial shaft arrangement by means of two or more actuators have been developed. Well known examples that have been used on turbojet engines for many years are the systems used to actuate variable angle compressor stator vanes. Variable angle compressor stator vanes are rotated as engine speed is varied to accomodate the varying rotor discharge vector angles. These vanes are simultaneously actuated by rotating unison rings which interconnect all the vanes by means of cranks attached to the vane shanks. The vane shanks project radially through the aircraft engine casing wall so that rotating the vane shanks causes all of the vanes inside the engine to rotate through an identical angle. Motion is initiated by two symmetrically arranged actuators that rotate the unison rings.
While this type of system is ideal for rotating multiple engine stator vanes with multiple radial shafts, it has limitations where relatively few radial shafts are to be rotated in unison, as in the case of the forward and rear VABI's on the variable cycle engine. In the variable cycle engine, it is desirable to use as few as three radial shafts to actuate the valve in order to eliminate weight and complexity. When fewer radial shafts are employed, the shafts are more widely separated physically, and it is more difficult to mechanically synchronize the rotation of the shafts.
An additional problem occurs on the forward sliding valve, because it is located in the forward portion of the aircraft engine where controls and accessories necessarily occupy a considerable portion of the underside of the engine casing. The unison ring employed in previous mechanisms circumferentially surrounds the entire engine casing and therefore would occupy part of the same space. Physical interference of the ring with the controls package would result in both increased size of a surrounding envelope and in maintenance problems due to difficulty involved in assembly or removal of an actuation ring inside the controls package. Still another problem results from the location of part of or all the valve actuating mechanism in the interior of a gas turbine engine. For instance, the location of such a mechanism in the outer bypass duct of a gas turbine engine adversely affects overall engine performance by constituting a partial flow obstruction to the fan air passing therethrough. Such a flow obstruction causes aerodynamic drag losses in the fan air which results in decreased engine performance.
Finally, it is desirable to employ an actuation system using only one actuator to save weight and reduce complexity.